This invention relates to an anhydrous process for the purification of alkyl aromatics obtained by the Friedel-Crafts reaction of aromatics with olefins in the presence of an aluminum chloride catalyst and isolation following precipitation of the catalyst complex with anhydrous ammonia.
Alkyl aromatics are produced commercially on a large scale by the Friedel-Crafts reaction. For example, ethylbenzene is made from benzene and ethylene with aluminum chloride as catalyst and hydrogen chloride as promoter. The anhydrous crude alkylate obtained by this reaction still contains catalyst complex and hydrogen chloride, both of which must be removed prior to distillation of the product. This is accomplished in a separating tank wherein the bulk of the heavier catalyst complex is separated by phase separation. In the method of Russian Pat. No. 309,570 and DOS (German Unexamined Laid-Open application) No. 2,365,175, residual catalyst complex in the crude alkylate after phase separation is converted by reaction with ammonia to a compound insoluble in the crude alkylate and is then separated mechanically.
In the process of Russian Pat. No. 309,570, gaseous ammonia is introduced into crude alkylate at a temperature between 70.degree. and 80.degree. C. According to the example given in DOS No. 2,365,175, the aluminum compound is precipitated after being cooled to 60.degree. C. This reference gives no data as to the quantity of ammonia utilized. In the example of Russian Pat. No. 309,570, ammonia is used in excess of 0.01-0.02 gram per liter of crude alkylate. The pH value is 9-10 in the aqueous extract of the crude alkylate. No data are given regarding pressure during ammonia addition in either Russian Pat. No. 309,570 or in DOS No. 2,365,175. Therefore, it is assumed that both processes are conducted at atmospheric pressure. The slightly soluble aluminum compounds formed under these conditions, i.e., a temperature of 60.degree. to 70.degree.-80.degree. C, respectively, pH of 9-10 in the aqueous extract (required because these prior art processes are performed under anhydrous conditions), at a pressure of 0 bar, however, have the grave disadvantage of being difficult to filter. The filtration rate is very slow. A further disadvantage of the known techniques is that, when ammonia is introduced at atmospheric pressure, the formation of ammonium chloride in the gas space above the reaction mixture is uncontrollable. Since ammonium chloride is sublimed at high temperatures, it will be deposited in pipes and apparatus, with consequent clogging and marked reduction of the heat transfer capacity of the distillation columns after a short period of time.
Therefore, there is a continuing need for a process for removing residual catalyst complexes from Friedel-Crafts alkylate by addition of anhydrous ammonia under anhydrous conditions without incurring these disadvantages and to produce easily filterable precipitates but no uncontrolled deposition of NH.sub.4 Cl.